High-order nonlinear multiphoton absorption is usually inefficient,but can be enhanced by designing resonant excitations between occupied and unoccupied energy levels.We conducted angle-resolved multi-photon photoemis...High-order nonlinear multiphoton absorption is usually inefficient,but can be enhanced by designing resonant excitations between occupied and unoccupied energy levels.We conducted angle-resolved multi-photon photoemission(mPPE)studies on the SnSe_(2)(001)surfaces excited by ultrashort laser pulses.By tuning photon energy and light polarization,we demonstrate the presence of a resonant four-photon photoemission(4PPE)process involving the occupied valence band(VB),the unoccupied second conduction band(CB2)and the unoccupied image-potential state(IPs)of SnSe_(2).In this 4PPE process,VB electrons of SnSe_(2) are resonantly excited into CB2 by adsorbing two photons,followed by the adsorption of another photon to populate the n=1 IPs before being emitted out to the vacuum by adsorbing one more photon.This results in a double-resonant 4PPE process,which exhibits approximately a 40 times enhancement in photoemission yields compared to cases where one of the resonant pathways,CB2→IPs,is inhibited by involving a virtual state instead of the IPs in the 4PPE.The double-resonant 4PPE process efficiently excite the bulk VB electrons outside the vacuum,like taking advantage of resonant“ladders”through two real empty electronic states of SnSe_(2).Our results highlight the important applications of mPPE in probing the band-structure,particularly the unoccupied states,of recently emerging main group dichalcogenide semiconductors.Furthermore,the discovered resonant mPPE process contributes to the exploration of their promising optoelectronic applications.展开更多
基金the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XD30000000)the National Key R&D Program of China(Grant Nos.2018YFA0305802 and 2017YFA0303500)the National Natural Science Foundation of China(Grant No.11774267).Calculations were performed at the supercomputing center of WHU of China.
文摘High-order nonlinear multiphoton absorption is usually inefficient,but can be enhanced by designing resonant excitations between occupied and unoccupied energy levels.We conducted angle-resolved multi-photon photoemission(mPPE)studies on the SnSe_(2)(001)surfaces excited by ultrashort laser pulses.By tuning photon energy and light polarization,we demonstrate the presence of a resonant four-photon photoemission(4PPE)process involving the occupied valence band(VB),the unoccupied second conduction band(CB2)and the unoccupied image-potential state(IPs)of SnSe_(2).In this 4PPE process,VB electrons of SnSe_(2) are resonantly excited into CB2 by adsorbing two photons,followed by the adsorption of another photon to populate the n=1 IPs before being emitted out to the vacuum by adsorbing one more photon.This results in a double-resonant 4PPE process,which exhibits approximately a 40 times enhancement in photoemission yields compared to cases where one of the resonant pathways,CB2→IPs,is inhibited by involving a virtual state instead of the IPs in the 4PPE.The double-resonant 4PPE process efficiently excite the bulk VB electrons outside the vacuum,like taking advantage of resonant“ladders”through two real empty electronic states of SnSe_(2).Our results highlight the important applications of mPPE in probing the band-structure,particularly the unoccupied states,of recently emerging main group dichalcogenide semiconductors.Furthermore,the discovered resonant mPPE process contributes to the exploration of their promising optoelectronic applications.
